It is notoriously difficult to design and construct a single loudspeaker to operate with sufficiently uniform efficiency over the entire range of frequencies which are normally required for the reproduction of sound with high quality. This range normally extends from approximately 30 to 15,000 Hz. Normally the loudspeaker system should exhibit such efficiency and power handling ability that a sufficient volume of sound can be generated with low distortion when the loudspeaker is used in conjunction with an ordinary power amplifier capable of delivering up to 50 watts. The loudspeaker should normallly distribute the middle- and high-frequency energy within a horizontal angle of .+-. 30.degree. to 40.degree. and a vertical angle of .+-. 20.degree. to 30.degree., referred to the normal radiating axis of the loudspeaker, so that within these limits serious variations of frequency response do not occur. Although the invention is not intended exclusively for use within these limits, the limits do define the normal conditions of operation.
Owing to the difficulty of constructing a single loudspeaker as aforesaid, it is ordinary practice to use at least two acoustic radiators, each of which is designed to respond optimally in a part of the total frequency range for which the production of acoustic wave signals is required. The electrical drive signals to the acoustic radiators are usually divided by complementary filters so that each acoustic radiator is fed with signals appropriate to its part of the frequency spectrum. The filters which are normally employed are usually the so-called "two-pole" filters. They normally exhibit a rate of change of attenuation, beyond a respective cut-off frequency of 12 decibels per octave of frequency. The ordinary response of the filters may be adjusted by a variety of expedients to compensate for deviations of the responses of the loudspeakers from that which is desired. The filters are normally arranged so that each attenuates the signals which pass to the respective acoustic radiator but are within the frequency range pertinent to the other acoustic radiator. The frequency above which signals are principally radiated by one of the radiators and below which are principally radiated by the other radiator is called the crossover frequency. For simplicity it is convenient to consider systems in which there are only two acoustic radiators and one crossover frequency but it is possible to fulfil the requirements for the loudspeaker system by using three or more acoustic radiators, sub-dividing the spectrum of electrical signals for feeding to the acoustic radiators accordingly and equalising the responses of the individual radiators constituting the complete loudspeaker system.
Very many attempts have been made, without conspicious success, to achieve satisfactory performance. It is accordingly the object of the present invention to provide an improved loudspeaker system and according to another aspect of the invention a loudspeaker system and a filter for use with it, and thereby to facilitate the attainment of uniform response over a frequency range by the use of two or more loudspeakers to radiate signals preferentially over a respective part of the frequency range.